In a video-on-demand system, "programs," such as games, movies, sporting events, concerts, etc., are typically supplied as analog signals. The analog signals can be recorded on magnetic or optical media, for example VHS tapes, for distribution. A "video server" can be used to transport the program signals on-demand to customer premises. The communication media used to transport the program signals can be public, as well as private.
To reduce the bandwidth required to transport the program signals over the communications, the analog signals can be encoded and compressed into digital signals, also known as "data." The compressed program data can deliver, for example, audio and video programs having a quality suitable for entertainment at rates around 1-2 Megabits per second (Mb/s).
According to the Motion Pictures Expert Group (MPEG) standard, the analog program signals are encoded as a transport stream. During the encoding, spatial and temporal compensation are used to compress the programs. If the program includes video signals, chromatic compression can also be used. Under the MPEG standard, the number of bits which represent a particular timed sequence of the program can vary because of variations in compression efficiency. Equipment at the customer's premises sequentially accesses the transport stream to decode, decompress, and reconstruct the program for replay on an audio-visual device, such as a television or PC.
Since the number bits of the encoded transport stream can very per unit of program time, it is important that the relative "real" time relationship of the encoded signals be maintained. The real time of the program is maintained by using program clock references (PCRs). The PCRs are added to the transport stream during encoding. The PCRs are used during decoding to present the reconstructed program at a rate in accordance with the real time of the original program.
According to the MPEG standard, the PCRs are encoded as forty-two bit values. In order to maintain an accurate real time of the program, the PCRs are added to the encoded transport stream at program time intervals not to exceed 100 milliseconds. Effectively, the PCRs periodically time-stamp the transport stream with the program's real time. Thus, the customer premises equipment can, time-wise, accurately reconstruct the analog program signals for play on audio-visual devices.
Great care must be taken in delivering the transport stream to the customer premises equipment at a rate which remains relatively constant with respect to the program's real time. If the transport stream is delivered too slow, the program falls behind. This would be perceived as a substantial deterioration of the program. If the transport stream is delivered too fast, buffers in the customer premises equipment used to store the transport stream during decoding would overflow, and portions of the program could be lost. The situation where there is gross time-wise displacement of the program is called "wander."
The MPEG standard specifies a system layer wherein the transport stream is partitioned into fixed size "packets." The MPEG standard requires that each transport stream (TS) packet transports exactly 188 bytes, each byte transporting eight bits of the program.
In a modem high-speed communications network suitable for transporting program signals, for example, a broadband integrated services network (BISDN), the digital signals are communicated at a frequency which is synchronized to a network clock. This means that the data are transported at a constant bit rate. The BISDN can include central offices (COs) interconnected by high-speed trunks. Typically, the trunks operate at standard rates, for example, frequencies which are multiples of the basic Synchronous Optical Network (SONET) transport rate defined by the International Consultative Committee for Telephony and Telecommunications (CCITT). For example the SONET STS-12c/OC-12 standard specifies a transport rate of 622 Mb/s.
The BISDN typically employs asynchronous transfer mode (ATM) techniques. The technique is called asynchronous because data are scheduled for transport on a basis of need and availability. In this technique, the signals or "data" are routed through the network over "virtual circuits" in self-contained, fixed-length quantifies called "cells." The circuits are virtual because successive network sessions between the same source and destination can use physically different routes.
Therefore, the cells, in addition to transporting "payload" data, also carry control and routing information. The control and routing information cause the ATM switches to direct the cells to their proper destination. Communications standards used with ATM techniques specify that cells will transport exactly 53 bytes. The first five bytes of the cell are dedicated to transporting the control and routing information. The last 48 bytes are available for transporting the payload data. If no data are available for transport over a circuit at any instant in time, an idle cell must be generated to maintain the constant bit rate.
To format 188 byte MPEG TS packets fit into ATM transport cells, it has been proposed that two TS packets, e.g., 376 bytes, be transported as an eight cell ATM adaptation layer five (AAL5) protocol data unit (PDU). The eight cells of the PDU provide 384 payload bytes. The remaining eight bytes of payload capacity are used to transport a common part convergence sub-layer adaptation trailer which is used by the customer premises equipment to reconstruct the TS packets.
A problem arises as a result of formatting the two TS packets into the eight cell PDUs. If a first TS packet of a PDU includes a PCR, then the customer premises equipment can not reassemble the first packet until after the second packet of the PDU has been received. This means that the PCR of the first TS packet is interpreted later than anticipated. This may cause the reconstructed program signals to be locally distorted in time. This localized time distortion is sometimes known as "jitter."
Therefore, is also proposed that a single 188 byte TS packet, plus the eight byte AAL5 trailer, be transported as a five cell PDU. Here, only 6 of the 240 available payload bytes are consumed. This means that 44 bytes, or about a little less than a fifth of the PDU do not transport timed program data.
Constructing and transporting cells which are only partially used consumes server resources and network band-width. Transporting five cell PDUs also delays the delivery of a next cell by the amount of time it takes to transport the additional 8.times.44 (352) bits which were not accounted for during encoding. If the number of five cell PDUs for a particular portion of the program is relatively high, then the reconstructed program wanders in time, if the program is transported using ATM type of networks.
It is desired to transport five and eight cell PDUs in a manner which does not cause distorting jitter and wander in the reconstructed program.
One solution is to analyze the program, prior to transport, and determine exactly the number and location of the PCRs. Then, the relative number of five and eight cell PDUs can be determined in order to allocate a virtual circuit having a bandwidth which closely matches the required transport rate.
A typical MPEG encoded movie program may approximately include 2.4 gigabytes of dam. Scanning and analyzing this amount of data consumes time. In an interactive environment, users can initiate the delivery of a large number of different programs in a relatively short time. For example, in a home-shopping session, the user may view catalogs, product demonstrations, and credit information. Pre-scanning all possible programs in real-time would interfere with the interactive delivery of the programs.
An alternative solution would adjust, as the TS packets are formatted into PDUs, the PCRs. However, now the transport stream may no longer be compliant with standard MPEG encoding. Furthermore, this solution, perhaps, makes the transport stream unsuitable for transport protocols, other than ATM, which do not use five and eight cell PDUs.
Therefore, it is desired to provide a method and apparatus which can condition the timed program data independent of transport timing to minimize jitter and wander during program reconstruction. Furthermore, the method and apparatus should operate without wasting system and network resources. In addition, it is desired that the method and apparatus operate on the program signals representing program data as they are being delivered in an interactive environment.